As early humans increasingly left forests and utilized¹ tools, they took an evolutionary² step away from apes. But what this last common ancestor with apes looked like has remained unclear. A new study led by researchers at UC San Francisco shows that important clues lie in the shoulder. Humans split from our closest African ape relatives in the genus Pan - including chimpanzees and bonobos - 6 to 7 million years ago. Yet certain human traits resemble the more distantly related orangutan or even monkeys. This combination of characteristics calls into question whether the last common ancestor of modern humans and African apes looked more like modern day chimps⁴ and gorillas⁶ or an ancient ape unlike any living group. 

"Humans are unique in many ways. We have features that clearly link us with African apes, but we also have features that appear more primitive⁷, leading to uncertainty⁸ about what our common ancestor looked like," said Nathan Young, PhD, assistant professor at UC San Francisco School of Medicine and lead author of the study. "Our study suggests that the simplest explanation, that the ancestor looked a lot like a chimp³ or gorilla⁵, is the right one, at least in the shoulder." 

It appears, he said, that shoulder shape tracks changes in early human behavior such as reduced climbing and increased tool use. The paper, titled Fossil Hominin Shoulders Support an African Ape-like Last Common Ancestor of Chimpanzees and Humans, published online Sept. 6, in the journal PNAS. 

The shoulders of African apes consist of a trowel-shaped blade and a handle-like spine⁹ that points the joint¹⁰ with the arm up toward the skull¹¹, giving an advantage to the arms when climbing or swinging through the branches. In contrast, the scapular spine of monkeys is pointed¹² more downwards¹³. In humans this trait is even more pronounced, indicating behaviors such as stone tool making and high-speed throwing. The prevailing¹⁴ question was whether humans evolved this configuration¹⁵ from a more primitive ape, or from a modern African ape-like creature, but later reverted¹⁶ back to the downward angle.